Various and numerous industrial processes exist wherein it is desirable and necessary to supply air for a particular processing use at precise conditions of dry bulb temperature and relative humidity. Thus for example, large helical-gear train machining operations frequently are carried out under precisely controlled ambient air conditions respecting both temperature and humidity. Similarly, reordering or moistening of relatively dry tobacco is effected with an air flow which is conditioned with exactitude respecting the temperature and humidity of air passed through the tobacco.
The precision required in such industrial applications particularly as pertaining to relative humidity control in an air stream flow has led to development of relatively high energy consuming air processing techniques to attain that precision. In other words, in order to condition air or reestablish particular parameters of dry bulb temperature and relative humidity in spent air involves energy expenditures for cooling and reheating which are far greater than the actual heat exchange or enthalpy change required to produce change of such parameters from a first spent condition to a second desired condition. Thus, e.g., known processing utilizes low velocity spray washer systems to cool and saturate air at a desired dew point temperature (DPT) to attain the required relative humidity at the desired dry bulb temperature followed by reheating of the air sensibly to the requisite dry bulb temperature. Both of these process steps use unnecessary energy and are only desirable and accepted because of the ease of measuring the temperatures accurately.
In a typical factory air conditioning system, by way of example, spent air, i.e., air that has been subjected to a particular use is returned to an air conditioning unit from a space such that there is an air change every 5 minutes, air handling being at the rate of 25,000 CFM. The return or spent air has gained sensible heat from electric motors, fans, radiation from walls and hot surfaces, and also latent heat and moisture from evaporation of body perspiration and process leaks to such an extent that the air is, e.g., at 77.degree. F dry bulb temperature (DBT) and substantially 59% RH. With reference to a psychrometric chart, it will be noted that such spent air has:
Specific Volume -- 13.75 ft..sup.3 /lb. of d.a. PA1 Dpt -- 61.5.degree. f PA1 Enthalpy -- 31.4 Btu/lb of d.a. PA1 Specific Volume -- 13.7 ft..sup.3 /lb of d.a. PA1 Dpt -- 60.1.degree. f PA1 Enthalpy -- 30.2 Btu/lb of d.a.
If it is desired to return the air to the room at designed control conditions of, e.g., 75.degree. F and 60% RH it will be seen from the psychrometric chart that such air would have:
From the foregoing data it will be noted that to reestablish the desired dry bulb temperature and relative humidity values in the air involves a minimum required heat exchange of 1.2Btu/lb of dry air to reduce the enthalpy to the desired level. However, measurement of enthaply can only be accomplished under strict laboratory conditions and, heretofore, continuous accurate measurement for control purposes of any condition other than dry bulb temperature has been difficult in attainment. For such purpose, resistance temperature detectors (RTD's) made of platinum have been developed to measure temperatures within 0.15.degree. F with a repeatability of 0.5.degree. F. Since a small percentage error in measurement of the dew point can cause a large change in relative humidity at a given dry bulb temperature, indirect means have been employed to insure correct measurement of the dew point.
Since it is relatively simple to measure the dry bulb temperature of air accurately and also the temperature of water using RTD's it heretofore has been common practice to reestablish desired conditions in the air by passing it through a washing operation wherein it is subjected to a plurality of sprays spraying water at a desired dew point temperature to effect heat exchange, e.g., remove heat from the air, and since the air leaving the washer has a DBT equal to the temperature of the water entering and leaving the last spray, the air is saturated (100% RH) at that DPT. Referring again to a psychrometric chart shows the enthalpy of saturated air at the desired DPT is 26.5 Btu/lb of d.a.
Thus it will be noted the prior art processing has removed 3.7 more Btu/lb than was required to attain the desired conditions and further the air must now be heated to resupply lost enthalpy and restore the air to desired relative humidity and DBT sensibly. This results in total excess of energy use equivalent to 7.4 Btu/lb of dry air in addition to the 1.2 Btu/lb enthalpy reduction required to bring the spent air to the desired conditions.
On the basis of 25,000 CFM flow for example at 13.7ft.sup.3 /lb of d.a. or 109,489 lb of d.a. per hour, the required air conditioning load, i.e., the enthalpy differential of dry air at the spent and desired conditions is 131,386 Btu/hr. and the energy wasted by prior art processing is 810,219 Btu/hr. or a wastage of about 86%.
It is seen that prior art processing or conditioning of air to provide desired dry bulb temperature and relative humidity conditions is wasteful of energy. In view of the current energy crisis it is important that processing of air for the purposes described above be effected in such manner as makes possible avoidance of wasteful and unnecessary usage of energy as has heretofore been experienced.
As has been discussed earlier the requirements for conditioning air with exactitude are numerous and hence the potential for energy savings in such processing is of major importance. Illustrative of one such requirement is in the tobacco industry, wherein the control of the moisture content of the air in cut filler storage operation and the cigarette making and packing operations has been shown to be of considerable importance. It is known that tobacco, a natural product, will gain moisture in the presence of high humidity air and lose moisture in the presence of low humidity, and that tobacco of a given grade and crop year will reach the same equilibrium moisture content when exposed to the same RH and DBT air for a suitable period of time and will remain at that moisture as long as the air conditions are maintained constant, a blend of tobacco behaving in this respect in the same manner as individual tobacco grades.
In the cigarette making process shredded tobacco is brought to an ideal or desired moisture content for making and packaging cigarettes. It is important to maintain that desired moisture content throughout the remainder of the processing to prevent breakage, flavor changes, adhering to equipment, and also to ensure final uniform quality of the packaged cigarette. To attain this end the storage, conveying, making, and packaging areas must be maintained at a constant equilibrium RH and DBT to maintain the desired moisture content.
It also is known that reordering (moistening) expanded tobacco can be accomplished by passing a moving bed of relatively dry expanded tobacco through a chamber where carefully controlled humidity air is passed through the bed to raise the moisture content of the tobacco to the proper level for storage, handling, blending, and cigarette making with minimal loss of filling power. The rate of moisture addition at certain moisture levels can affect the filling power of the expanded tobacco. For such purposes, the tobacco could be exposed to its ideal equilibrium moisture humidity air for a day or two to effect the slowest moisture transfer to the tobacco and thus little or no loss of filling power.
Practically, for commercially feasible production requirements it is desirable to accomplish the reordering in a short time such as 30 minutes, which is an attainable goal since it is known that the majority of the moisture to be added to the tobacco can be done fairly rapidly by subjecting the tobacco to a higher RH air up to a certain tobacco moisture and then treating the tobacco with a lower humidity air to add the last few percent moisture. In reordering or moistening relatively dry tobacco it will be understood that the end aim or purpose is to increase the moisture content to that requisite for optimized commercial handling of tobacco as noted above. As used herein "relatively dry tobacco" is meant tobacco containing moisture at a level substantially below that required for processing thereof. In the case of expanded tobacco the desired moisture content should be about 11% but as an incident of expansion the moisture content will have been lowered to about 2%. Cut natural blend or otherwise unprocessed tobacco on the other hand should have a processing moisture content of about 131/2%.